Waveguide dual-depletion region (ddr) photodiodes

ABSTRACT

Consistent with the present disclosure, a DDR photodiode is provided on a substrate adjacent to a passive waveguide. In order to efficiently capture light output from the waveguide, the photodiode is coupled to the waveguide with a butt-joint. As a result, the photodiode and the waveguide abut one another such that the dominant mode of light propagating in the waveguide parallel to the substrate is supplied directly to a side of the absorber layer of the photodiode without, in one example, evanescent coupling, nor is a resonant coupler required to supply light to the photodiode. Thus, light is absorbed more efficiently in the photodiode such that the photodiode may have a shorter length. In addition, since substantially all light is input to the photodiode, nearly complete absorption and nearly ideal quantum efficiency can be achieved in a relatively short length. Further, the improved linearity associated with DDR photodiodes is preserved with the exemplary butt joint configurations disclosed herein.

BACKGROUND

Optical communication systems are known whereby one or more opticalsignals, each being modulated to carry information, are transmitted froman optical transmitter to an optical receiver. The optical receiverstypically includes, among other things, one or more photodiodes thatconvert the received optical signals into corresponding electricalsignals, which are then further processed. In certain opticalcommunication systems, various components or devices in the receiver areintegrated on a common substrate as a photonic integrated circuit (PIC).Such components include optical waveguides and photodiodes that, in someinstances, receive light supplied by the optical waveguides.

So-called dual-depletion region (DDR) photodiodes are known in which an“absorber” layer is located above an undoped layer of bandgap wider thanthe absorber, in such a manner that photo-generated holes have a shorterdistance to travel to the p-type region anode above the absorber, whilephoto-generated electrons have a longer distance to travel to the n-typeregion cathode below the undoped layer. The depletion region extendsfrom above the absorber layer to below the lower undoped layer. Sinceelectron mobility is much higher than hole mobility in InP and relatedmaterials, the total transit time of such holes and electrons from theabsorber to their respective contacts is comparable or minimized. Asimilar photodiode without a undoped layer below the absorber would havemuch higher capacitance, and a similar photodiode with an absorber asthick as the two undoped layers can absorb too much light at the inputso that the high speed photocurrent response is nonlinear. Accordingly,carrier lifetime in the DDR photodiode is reduced, and the length andcapacitance of the photodiode may be optimized for both highresponsivity and high radio frequency (RF) bandwidth.

Conventional DDR photodiodes detect light that is incident at adirection that is normal to the substrate upon which the DDR photodiodeis provided. PICs, however, include optical waveguides that confineoptical signals, whereby the optical signals propagate in the opticalwaveguides in a direction parallel to the substrate. Accordingly,conventional DDR photodiodes may not be suitable for integration in aPIC, and the associated benefits of such photodiodes may be difficult toobtain in optical receivers including PICs.

SUMMARY

Consistent with an aspect of the present disclosure, an optical receiveris provided that comprises a substrate and an optical waveguide having acore layer provided on a first region of the substrate. The receiveralso includes a photodiode provided on a second region of the substrate,such that an interface between the optical waveguide and the photodiodeconstitutes a butt joint. The photodiode includes a first semiconductorlayer having a p-conductivity type, the first semiconductor layer beinga p-type cladding layer. The photodiode also includes a secondsemiconductor layer having n-conductivity type, the second semiconductorlayer being an n-type cladding layer. Further, the photodiode includesan absorber layer provided between the p-type cladding layer and then-type cladding layer. The absorber layer has a first undopedsemiconductor layer, such that the absorber layer is aligned with thecore layer of the optical waveguide to receive, via the interface, anoptical signal propagating in the optical waveguide. Moreover, thephotodiode includes a second undoped semiconductor layer providedbetween the absorber layer and the second semiconductor layer, such thatin an absence of a reverse bias applied to the photodiode, a firstdepletion region forms in the absorber layer and a second depletionregion forms in the second undoped semiconductor layer.

Consistent with a further aspect of the present disclosure, an opticalreceiver is provided that comprises a substrate and an optical waveguideprovided on a first region of the substrate. In addition, the opticalreceiver includes a photodiode provided on a second region of thesubstrate, such that an interface between the optical waveguide and thephotodiode constitutes a butt joint. Further, the photodiode isconfigured to receive an optical signal supplied by the opticalwaveguide, wherein the optical signal has a propagation direction in theoptical waveguide. The interface between the optical waveguide and thephotodiode is provided at a non-orthogonal angle relative to thedirection of propagation of the optical signal.

Consistent with an additional aspect of the present disclosure, anoptical receiver is provided that comprises a substrate and an opticalwaveguide provided on a first region of the substrate. Further, aphotodiode provided on a second region of the substrate, such that aninterface between the optical waveguide and the photodiode constitutes abutt joint. The photodiode is configured to receive an optical signalsupplied by the optical waveguide, wherein the optical signal propagatesin the optical waveguide in a propagation direction, and the opticalsignal propagates in the photodiode in the same propagation direction. Awidth of the optical waveguide increases in the propagation direction.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments and togetherwith the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical receiver employing direct detection consistentwith an aspect of the present disclosure;

FIG. 2 a shows an optical receiver employing coherent detectionconsistent with an additional aspect of the present disclosure;

FIG. 2 b shows an example of a balanced (differential) detectorconsistent with the present disclosure;

FIGS. 3 a-3 c show plan view of waveguide-photodiode combinationsconsistent with aspects of the present disclosure.

FIG. 4 shows a perspective view of a waveguide-photodiode configurationshown in FIG. 3 a;

FIG. 5 a shows a cross-sectional view of photodiode and waveguideconsistent with a further aspect of the present disclosure;

FIG. 5 b shows a cross-sectional view of photodiode and waveguideconsistent with an additional aspect of the present disclosure;

FIG. 5 c shows a cross-sectional view of photodiode and waveguideconsistent with a further aspect of the present disclosure;

FIG. 6 shows an example of an energy band diagram consistent with anadditional aspect of the present disclosure;

FIG. 7 is a plot of mode overlap ratio with a passive waveguide vs.absorber thickness consistent with an aspect of the present disclosure;

FIG. 8 shows plots of total harmonic distortion vs. bias voltageconsistent with an additional aspect of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Consistent with the present disclosure, a DDR photodiode is provided ona substrate adjacent to a passive waveguide. In order to efficientlycapture light output from the waveguide, the photodiode is coupled tothe waveguide with a butt-joint. As a result, the photodiode and thewaveguide abut one another such that the dominant mode of lightpropagating in the waveguide parallel to the substrate is supplieddirectly to a side of the absorber layer of the photodiode without, inone example, evanescent coupling, nor is a resonant coupler required tosupply light to the photodiode. Thus, light is absorbed more efficientlyin the photodiode such that the photodiode may have a shorter length. Inaddition, since substantially all light is input to the photodiode,nearly complete absorption and nearly ideal quantum efficiency can beachieved in a relatively short length. Further, the improved linearityassociated with DDR photodiodes is preserved with the exemplary buttjoint configurations disclosed herein.

Reference will now be made in detail to the present exemplaryembodiments of the disclosure, which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 1 shows a high level circuit diagram of receiver 104 consistentwith an aspect of the present disclosure. Receiver 104 includes awaveguide 194 that carries optical signals which may be amplitudemodulated. The optical signals are supplied to photodiode 192, which maybe appropriately biased. In one example, the photodiode is reversedbiased such that a positive reference or bias voltage +Vdd is suppliedto cathode 192-2 and a negative reference or bias voltage −Vdd issupplied to anode 192-1.

FIG. 2 a shows coherent optical receiver 104 c, which is another exampleof an optical receiver consistent with the present disclosure. In oneexample, a polarization multiplexed optical signal is supplied toreceiver 104C. In that case, receiver 104 may include a polarizationbeam splitter (PBS) 202 operable to receive the input optical signal andto separate the signal into orthogonal polarizations, i.e., vectorcomponents of the optical E-field of the incoming optical signaltransmitted on optical fiber medium 108, into signals X and Y. One ofthe polarizations is parallel to the local oscillator (LO) polarization,and the other is rotated to be parallel to the LO (201). The LO outputis split by an optical coupler (203). X and Y light are then each mixedwith portions of the LO output in their own 90 degree optical hybridcircuits (“hybrid”) 204-1 and 204-2. Hybrid 204-1 outputs four opticalsignals O1 a, O1 b, O2 a, and O2 b, and hybrid 204-2 outputs fouroptical signals O3 a, O3 b, O4 a, and O4 b, each representing thein-phase and quadrature components of the optical E-field of X and Ysignals, and each including light from local oscillator 201 and lightfrom polarization beam splitter 202 or mixing products. Optical signalsO1 a, O1 b; O2 a, O2 b; O3 a, O3 b; and O4 a, O4 b are supplied torespective one of photodetector circuits 209, 211, 213, and 215. Eachphotodetector circuit includes a pair of photodiodes (“2PDs”) configuredas single ended or else as balanced detector, as shown in the exampleFIG. 2 b.

FIG. 2 b shows photodetector circuit 209 in greater detail. It isunderstood that remaining photodetector circuits 211, 213, and 215 havea similar construction and operate in a similar manner as photodetectorcircuit 209. As shown in FIG. 2 b , optical waveguides 194-1 and 194-2supply optical signals O1 a and O1 b, respectively, from optical hybrid204-1 to a corresponding one of photodiodes 292 and 294. Photodiodes 292and 294 are connected in a balanced detector configuration, and theoutput, E1, of photodetector circuit 209 is supplied to a transimpedanceamplifier (TIA) circuit, whose output then supplies an analog-to-digitalconversion (ADC) circuit for further processing.

Each of remaining photodetector circuits 211, 213, and 215 generates acorresponding one of electrical signals E2 to E4 in a similar manner asthat described above with respect to photodetector circuit 209. SignalsE2 to E4 are also supplied to respective TIA/ADC circuits. Electricalsignals E1 to E4 are indicative of data carried by optical signals inputto PBS 202.

FIGS. 3 a to 3 c show plan views or layouts of examples of passivewaveguide-photodiode combinations consistent with the presentdisclosure. As shown in FIG. 3 a an optical signal represented byoptical mode 302 propagates in direction 304 in waveguide 194. Waveguide194 has a width transverse to direction 304 that increases in direction304. For example, at location L1, waveguide 194 has a width w1, and, atlocation L2, waveguide 194 has a width w2 that is greater than width w1.Location L1 is farther away from interface 306 than location L2.

As further shown in FIG. 3 a , interface 306 is present between opticalwaveguide 194 and photodiode 192. In one example, interface 306constitutes a butt joint, whereby photodiode 194 is configured toreceive optical mode 302 directly from optical waveguide 194. Moreover,in the example shown in FIG. 3 a , interface 306 is oriented or providedat a non-orthogonal angle α relative to direction 304. Angle α may havea magnitude that is greater than or equal to 5° and less than or equalto 85°.

Optical mode or signal 304 next propagates into photodiode 192 and isabsorbed along a length of photodiode 192. In one example, a width ofphotodiode 192 narrows in a direction corresponding to propagationdirection 304, such that at location L3 photodiode 192 has a width w3,which is greater than a width w4 of photodiode 192 at location L4.Location L3 is nearer interface 306 than location L4.

In another example, as shown in FIG. 3 b , the width w5 of photodiode192 is uniform along a length extending in a propagation direction 304.However, the width of waveguide 194 increases in a directioncorresponding to propagation direction 304, as shown in FIG. 3 a.

In a further example, as shown in FIG. 3 c , both waveguide 194 andphotodiode 192 have the same width w6, and such width is uniform inpropagation direction 304.

FIG. 4 shows a perspective view of waveguide 194 and photodiode 192corresponding to the configuration shown in FIG. 3 a.

FIG. 5 a shows a view of waveguide 194 and photodiode 192 taken alongcross-section 5 (see FIG. 3 a ) adjacent interface 306. It is understoodthat, in one example, waveguide 194 and photodiode 192 shown in FIGS. 3b and 3 c will have a similar construction as that shown FIG. 5 a , and,in further examples, waveguide 194 and photodiode 192 have thestructures shown in FIGS. 5 b and 5 c.

As shown in FIG. 5 a , waveguide 194 is provided on region 594 ofsubstrate 404, and photodiode 192 is provided on region 592. N-typecladding layer 512, in one example, may be provided on substrate 402that extends over both regions 594 and 592. The n-type cladding layerincludes, in a further example, indium phosphide (InP). An additionaln-type or n-doped layer 511 may be provided on a first cladding layer512. Layer 511 may also extend over both region 594 and region 592.Layer 511 may also include InP.

As further shown in FIG. 5 a , waveguide 194 provided over region 594includes, in one example, a core layer 610, which includes a quaternarysemiconductor material, such as, indium gallium arsenide phosphide(InGaAsP) or other suitable quaternary semiconductor material, such asAlGalnAs. Layer 610 is typically undoped or else lightly doped (≤1E17)n-type. As used herein, undoped means unintentionally doped or a dopingconcentration that is less than or equal to 1×1016. Waveguide 194 mayalso include undoped or n− layer 508, which is further provided onregion 594, and a layer 506 provided on layer 508. Layer 508 may beundoped or lightly doped n−. Waveguide 194 provided over region 594further includes a second cladding layer 504, which, in one example, isdoped p-type or implanted with H or He to be semi-insulating, and, infurther example includes InP.

Photodiode 192, as noted above, is formed over region 592 of substrate404. Photodiode 192 may be a DDR photodiode including an absorber layer.In the example shown in FIG. 5 a , photodiode 192 includes absorberlayer 522, which includes, for example, indium gallium arsenide(InGaAs). In a further example, absorber layer 522 is undoped orundoped. Photodiode 192 further includes cladding layer 520, which maybe p-type and includes InP. Cladding layer 520 is provided on absorberlayer 522. In addition, contact layer 518, which is also p-type andincludes InGaAs, may be provided on p-type cladding 520. As furthershown in FIG. 5 a , an additional contact layer 516 including aconductor or metal may be provided on contact layer 518.

As noted above, the absorber layer, such as layer 522 of photodiode 192in configured with undoped layer 508 below or has a thickness orelectron/hole mobility combination such that photo-generated holes havea shorter distance to travel to the p-type anode (520) whilephoto-generated electrons have a longer distance to travel to the n-typecathode (511) or photodiode 192. As further noted above, since theelectron mobility is higher than the hole mobility for InP and relatedmaterials, the transit time of such holes and electrons is substantiallythe same. Accordingly, carrier lifetime in the photodiode 192 isreduced, and radio frequency (RF) bandwidth is increased.

Moreover, absorber layer 522 is provided in a manner to be aligned withand abuts core layer 610 of waveguide 192, such that light isefficiently input to absorber layer 522 via interface 306 with minimalloss.

FIG. 5 b shows an example similar to that shown in FIG. 5 a . In Fib. 5b, however, optical core layer 510 extends over region 592 andconstitutes part of photodiode 592. In the example shown in FIG. 5 b anoptical mode or optical signal 503 propagates in a direction indicatedby arrow 514 in core 510. In photodiode 592, a tail portion 503-1 ormode 503 extends into absorber layer 522, and this generateselectron-hole pairs in a manner similar to that described above. In FIG.5 b , mode 503 is evanescently coupled to absorber 522.

FIG. 5 c shows an example similar to that shown in FIG. 5 b . In FIG. 5c , however, photodiode 192 includes an undoped layer 524 and an undopedor n-doped layer 526 provided between layer 508 and absorber layer 522.Layer 524 is provided on quaternary layer 526, in this example. In thisexample, layer 524 includes, for example, InP that is undoped and layer526 includes n-type InP.

In the examples shown in FIG. 5 c , optical mode or optical signal 503propagates in waveguide 194 in the direction indicated by arrow 514.Layers 524 and 526 are configured to facilitate a quaternary modetransfer from core 510 to absorber 522 by way of resonate coupling. As aresult, mode 503 generates electron-hole pairs in absorber layer 522 ina manner similar to that described above.

In each of the examples shown in FIGS. 5 a-5 c , appropriate biases areapplied to contact 516 and a contact, for example, to substrate 402 sothat the above-described electron-hole pairs generate a photocurrentindicative of the optical signal 503.

FIG. 6 shows an example of an energy band diagram 600 consistent with afurther aspect of the present disclosure. As shown in FIG. 6 , energyband diagram 600 includes regions 712, 711, 722, and 720 correspondingto the energy bands present in layers 511, 508, 522 and 520 of FIG. 5 a. Energy band diagram 600 further includes “band smoothing” regions 602and 604, which, in one example, includes a layer or layers comprising aquaternary semiconductor alloy, such as InxGa1-xAsyP1-y, with a bandgapintermediate between that of the InGaAs absorber and InP thatfacilitates carrier transit. Region 602 may be provided between InPlayer 508 and absorber layer 522, and region 604 may be provided betweenabsorber layer 522 and InP layer 520 in FIG. 5 a . In one example, theconcentration of phosphorus in the quaternary alloy InxGa1-xAsyP1-yvaries continuously along a thickness of region 602 and varies along athickness of region 604, e.g., in a direction away from the substrate(along the x-axis in FIG. 6 ). Alternatively, the smoothing region 602and 604 includes one or more layers, including a ternary semiconductoralloy or composition, and each layer within the band smoothing regionmay have a discrete composition and corresponding bandgap. Thus, bandsmoothing region 602 may include either a quaternary or ternarysemiconductor alloy and band smoothing region 604 may include either aquaternary or ternary semiconductor alloy.

As further shown in FIG. 6 , band smoothing region 604 is providedbetween region 720 (corresponding to layer 520) and region 722(corresponding to absorber layer 522). A valence band edge Ev withinregion 604 has an associated first energy that is between a secondenergy associated with a valence band edge Ev within region 722 and athird energy associated with a valance band edge Ev within region 720.In addition, band smoothing region 602 is provided between region 722and region 711 (corresponding to layer 508). A conduction band edge Ecwithin region 602 has an associated fourth energy that is between afifth energy associated with a conduction band edge Ec within region 722and a sixth energy associated with a conduction band edge Ec withinregion 711.

In each of the above example, the concentration of phosphorus in theband smoothing regions 602 and 604 may vary along a thickness of suchregions, e.g., in a direction along the x-axis in FIG. 6 .

In a further example, each of band smoothing regions 602 and 604includes AlGalnAs.

FIG. 7 is a plot 700 of the overlap ratio of the optical mode inphotodiode 192 to the optical mode in waveguide 194 vs absorberthickness for FIG. 5 a . FIG. 7 shows an example in which an absorberthickness of 0.1 microns results in a maximum amount of overlap of theoptical modes propagating in waveguide 192 and photodiode 194, nearly100%.

FIG. 8 illustrates plots 800 of total harmonic distortion (THD) for twoDDR photodiodes consistent with aspects of the present disclosure. Theseplots were generated in connection with an example in which light outputfrom two lasers was mixed to obtain a beating signal having a 1 GHzbeating tone. The beating signal was received by a photodiode consistentwith the present disclosure, such as photodiode 192. The total harmonicdistortion may be defined as the ratio between the total power of theharmonics (2 GHz, 3 GHz, etc.) over the RF power at 1 GHz. Opticalsignals incident from the two lasers provide photocurrents of 4.6 mA and0.4 mA, respectively. The dashed curved in FIG. 8 represents the THD asa function of bias voltage of a photodiode having band smoothing layerssimilar to those described above. The solid curve represent the THD as afunction of bias voltage of a photodiode without smoothing layers. Thedashed curve indicates that for a given bias voltage, the THD, and thusthe linearity is greater for the photodiode having smoothing layers,such as a band smooth layer between the p-type core layer 520 andabsorber layer 522, and between 522 and 508 below, than that of thephotodiode without such smoothing layers. The measurements depicted inFIG. 8 were obtained from photodiodes with a 3 micron input width.

In a further example, a responsivity of 1.1. NW was measured inconnection with a 25 micron long photodiode.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1. An optical receiver, comprising: a substrate; an optical waveguideprovided on a first region of the substrate; and a photodiode providedon a second region of the substrate, such that an interface between theoptical waveguide and the photodiode constitutes a butt joint, thephotodiode includes: a first semiconductor layer having a p-conductivitytype, the first semiconductor layer being a p-type cladding layer, asecond semiconductor layer having n-conductivity type, the secondsemiconductor layer being an n-type cladding layer, an absorber layerprovided between the p-type cladding layer and the n-type claddinglayer, the absorber layer including a first undoped semiconductor layer,such that the absorber layer is aligned with the core layer of theoptical waveguide to receive, via the interface, an optical signalpropagating in the optical waveguide, a second undoped semiconductorlayer provided between the absorber layer and the second semiconductorlayer, such that, in an absence of a reverse bias applied to thephotodiode, a first depletion region forms in the absorber layer and asecond depletion region forms in the second undoped semiconductor layer,a first band smoothing region provided between the p-type cladding andthe absorber layer, the first band smoothing region including a firstquaternary semiconductor alloy, and a second band smoothing regionprovided between the n-type cladding and the absorber layer, the secondband smoothing region including a second quaternary semiconductor alloy,wherein the photodiode is configured to receive an optical signalsupplied by the optical waveguide, the optical signal having apropagation direction in the optical waveguide, such that the interfacebetween the optical waveguide and the photodiode is provided at anon-orthogonal angle relative to the direction of propagation of theoptical signal.
 2. An optical receiver in accordance with claim 1,wherein the first and second quaternary semiconductor alloys includeindium gallium arsenic phosphide.
 3. An optical receiver in accordancewith claim 2, wherein a concentration of phosphorus in the firstsmoothing region changes along a thickness of the first smoothingregion, and a concentration of phosphorus in the second smoothing regionchanges along a thickness of the second smoothing region.